The UDP-glucuronosyltransferases (UGTs) are a family of enzymes involved in the metabolism of many drugs and xenobiotic compounds, as well as endogenous compounds, such as bilirubin, steroid hormones, bile acids, and thyroid hormones. Information on one important class of physiological compounds, retinoid glucuronides, and the enzymes involved in the glucuronidation of retinoic acid and related retinoids is limited. Specifically, the identity of the UGTs that have specificity toward retinoids have not been identified. One of the principal goals of this proposal is to elucidate mechanisms of formation and the chemical structures of the retinoid glucuronides. Specifically, this work will critically test the hypothesis predicting the presence of retinoid UGT enzymes in hepatic microsomes. It is postulated that they are either entirely novel isoforms or among the existing UGTs with unidentified substrate specificity for retinoids. In order to investigate this hypothesis, the following specific aims are proposed: 1) characterize retinoid glucuronidating activity in rat liver microsomes; 2) identify and isolate retinoid UGT protein(s) from rat liver microsomal fractions; and 3) identify available or novel recombinant UGT isoenzymes for retinoid glucuronidation activities.